This invention relates to a spectrophotometer of the type that the diffraction grating is driven directly by a motor.
A sine bar mechanism is known as a device to drive a diffraction grating used as dispersing means in a monochromator. This mechanism has an advantage that the amount of rotation of the screw rod for driving the sine bar, that is, the amount of rotation of the motor for driving the screw rod is in a linear relation to the wavelength of the light emerging from the monochromator. The mechanism, however, has a disadvantage that the structure is complicated.
In recent years electronic computers have come to be widely used to control spectrophotometers. In this case the amount of rotation of the motor for driving the diffraction grating is not in a linear relation to the wavelength. However, since the wavelength can be calculated from the amount of rotation of the driving motor, a system for driving the diffraction grating directly by a motor has come to be in increasing use, with an advantage that the mechanical structure is much simplified.
Generally, the grating pitch, that is, the spacing between ruled lines or grooves on the diffracting grating differs more or less from one diffraction grating to another. In those spectrophotometers which employ a sine bar mechanism for driving the diffraction grating, the above-mentioned difference is corrected by adjusting the length of the sine bar so that there is an exact correspondence between the amount of rotation of the motor and the wavelength obtained.
The system for driving the diffraction grating directly by a motor has no such means for mechanically correcting the above-mentioned difference in the grating pitch, so that if the grating pitch of a diffraction grating differs from the design or nominal value, the indication of wavelength contains an error. Thus the spectrophotometers of that type have been evaluated as having a low degree of accuracy in wavelength selection.